Third generation partnership project (3GPP) release 7 specifications introduce continuous packet connectivity (CPC) enhancements. CPC is a mode that a user equipment (UE) is allocated with resources, but transmits and/or receives discontinuously, (i.e., transmits and/or receives only in a subset of uplink and downlink sub-frames), during data inactivity while continuous operation is possible, (i.e., may transmit and receive in all sub-frames), whenever needed. The discontinuous uplink transmission and downlink reception increases system capacity and battery life of the UE.
For implementing the CPC, a higher layer, (e.g., radio resource control (RRC) layer), defines parameters for discontinuous transmission (DTX) and discontinuous reception (DRX) to set up the transmission and reception patterns and triggers. One of the parameters is “UE_DTX_DRX_Offset” that jointly controls an uplink dedicated physical control channel (DPCCH) transmission offset and a downlink high speed shared control channel (HS-SCCH) reception offset in sub-frames. By controlling the offsets of each UE, the network can ensure that the transmission and reception instants of different UEs are adequately spread over time.
DTX and DRX may be used for different applications. One of the applications is voice over Internet protocol (VoIP). With VoIP, the UE may be inactive in reception and/or transmission during a significant fraction of the time not only during uplink and downlink silence periods but also during voice active periods under certain conditions.
On the uplink, enabling DTX results in the UE transmitting the uplink DPCCH only according to the transmission pattern provided that certain conditions are satisfied, (e.g., no enhanced dedicated channel (E-DCH) transmission, no need to transmit pre-amble or post-amble, no high speed dedicated physical control channel (HS-DPCCH) transmission, and the like). The transmission pattern is defined based on the UE_DTX_DRX_Offset and a connection frame number (CFN) of the UE. Enabling DTX results in a direct capacity improvement since less interference is generated on the uplink. Such reduction of interference is achieved even during voice-active periods on the uplink if the UE is sufficiently close to a Node-B to be able to use a 2 ms transmission time interval (TTI), because not all hybrid automatic repeat request (HARQ) processes need to be used with the voice application with the 2 ms TTI.
On the downlink, enabling DRX results in the UE turning on its receiver only during user or control data reception and for a defined period afterwards, after which the UE is allowed to turn its receiver off according to a reception pattern. This allows the UE to save battery during voice-inactive periods on the downlink. This also allows UEs that are sufficiently close to the cell center to turn off their receivers intermittently during voice-active periods, since these UEs may receive at high instantaneous data rates and therefore during a fraction of the available TTIs. For instance, the RRC may configure DRX with the parameter Inactivity_Threshold_for_UE_DRX_cycle set to 0 sub-frame and the UE_DRX_cycle parameter set to 4 sub-frames. Thereafter, the Node B may schedule transmission for the UE only during the sub-frames where the UE should wake-up according to the pattern (1 out of 4).
Periods of activity and inactivity in a typical voice session alternate at a frequency that is generally too high to be tracked by RRC signaling without incurring unacceptable signaling load and delays. In accordance with one of the CPC implementation proposals, the UE_DTX_DRX_Offset is set by a higher layer, such as RRC. This means that during the course of a voice connection, the UE_DTX_DRX_Offset parameter remains to a fixed value.
Such restriction may reduce significantly the potential battery savings obtainable by the use of DRX on the downlink when the cell is operating near the maximum capacity because the use of DRX constrains the Node-B to schedule the UE during certain subsets of TTI only. As periods of voice activity and inactivity are not correlated between UEs, it will frequently happen that there will be an above-average number of voice-active UEs that can be scheduled for transmission in a given set of periodically spaced TTIs. Such situation would result in congestion (high latency) for the concerned UEs unless the Node-B disables the DRX for some of these UEs to schedule their transmissions in other TTIs.
On the uplink, the fact that the UE_DTX_DRX_Offset parameter is fixed is sub-optimal from the system capacity point-of-view because the system capacity may only be maximized if the set of voice-inactive UEs have staggered transmission patterns (with approximately equal number of voice-inactive UEs transmitting the DPCCH at a given time). This situation cannot be realized with a fixed UE_DTX_DRX_Offset since the set of voice-inactive UEs changes dynamically.